1. Field of the Invention
The present invention relates to a vibration wave motor for frictionally driving a movable member by a travelling vibration wave, and more particularly, to a drive device for the vibration wave motor.
2. Description of the Prior Art
FIG. 1 shows an example of a vibration wave motor driven by a travelling vibration wave, which has recently been put into practice. Numeral 1 denotes an electrostrictive element such as PZT (titanium acid zirconium acid lead), and numeral 2 denotes a vibration member made of elastic material to which the electrostrictive elements 1 are bonded. The vibration member 2 together with the electrostrictive elements 1 are held on a stator (not shown). Numeral 3 denotes a movable member which is press-contacted to the vibration member 2 to form a rotor. A plurality of electrostrictive elements 1 are bonded, and one group of elements thereof are arranged at a pitch shifted by one quarter of wavelength .lambda. of the vibration wave relative to the other group of elements. In each group, the electrostrictive elements are arranged at a pitch of one half of the wavelength with the polarities of adjacent elements being opposite to each other.
When a voltage Vo sin .omega.t is applied to one group of electrostrictive elements and a voltage of Vo cos .omega.t is applied to the other group, a bending vibration is generated in the vibration member 2, which vibration becomes a travelling wave and propagates in the vibration member 2 in a direction X.sub.1 of FIGS. 2(a)-2(d). Since the movable member 3 is contacted to the vibration member 2, peaks of the travelling wave of the vibration member contact to the lower surface of the movable member 3 to move the movable member 3 in the direction X.sub.2. Since the vibration wave motor of this principle generates a larger torque at a lower rotation speed than a conventional electromagnetic motor, reduction gears are not necessary. However, it has a drawback which is described below.
In this motor, an amplitude of the travelling vibration wave of the vibration member 2 is usually 2-3 .mu.m. Accordingly, if the contact surface of the vibration member 2 or the movable member 3 includes roughness, the movable member 3 is not always urged to the peaks of the vibration wave but is also urged to valleys of the vibration wave. Since the mass point at the valley of the vibration wave has the opposite velocity component to the mass point at the peak (see FIG. 2), and if the movable member simultaneously contacts the peak and the valley of the vibration amplitude, the left and right frictional drive forces cancel each other and a drive efficiency is reduced. Even if the contact surfaces of the movable member 3 and the vibration member 2 are good, good drive is not attained if dust of comparable size to the vibration amplitude is introduced into the clearance between both surfaces. Since the amplitude is substantially proportional to an applied A.C. voltage, it appears that the vibration amplitude increases as the voltage is increased and the above problem is resolved. However, as the vibration amplitude increases, the motion of the mass point on the vibration member 2 becomes violent and the speed increases. As a result, the motor cannot be driven at a low speed. Further, it is not preferable from durability standpoint of the element to apply a high voltage a high voltage to the electrostrictive element for a long period of time. If a frequency of the A.C. voltage applied to the electrostrictive element is lowered, the above problem is resolved. However, the frequency of the A.C. voltage is usually 20K-30K Hz, and if it is lowered to an audio frequency band, the vibration of the vibration member is audible to human beings and causes a noise. Thus, the frequency of the A.C. voltage applied to the electrostrictive element cannot be lowered lower than 20 KHz and it is difficult to drive the vibration wave motor at a low speed.
The vibration wave motor usually requires the A.C. voltage sources. The present assignee proposed a technique to drive the vibration wave motor by a D.C. voltage instead of the A.C. voltage (as described in commonly assigned U.S. patent application Ser. No. 552,373, filed Nov. 16, 1983), but it merely applies a chopped D.C. voltage instead of the A.C. voltage and does not resolve the above problem.